System and method for locating a mobile unit within the service area of a mobile communications network

ABSTRACT

A differential positioning system ( 10 ) includes components of a satellite-based or land-based positioning system ( 12 ) and components of a mobile communications network ( 14 ). The differential positioning system ( 10 ) provides accurate and immediate position information to a mobile unit ( 17 ). A transmitter site ( 40 ) of a mobile communications network ( 14 ) is associated with a reference positioning receiver ( 38 ). The reference positioning receiver ( 38 ) generates correction data for transmission to the mobile unit ( 17 ). The mobile unit ( 17 ) includes a mobile communications device ( 42 ) for receiving the correction data generated by the reference positioning receiver ( 38 ) and a mobile positioning receiver ( 24 ) for generating a position fix. The mobile unit ( 17 ) refines the position fix generated by the mobile positioning receiver ( 24 ) using correction data received by the mobile communications device ( 42 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. application Ser. No.08/340,755, filed Nov. 16, 1994, by Larry C. Wortham and entitled“Locating System and Method Using a Mobile Communications Network.”

TECHNICAL FIELD OF THE INVENTION

This invention relates to locating systems, and more particularly to alocating system and method using a mobile communications network.

BACKGROUND OF THE INVENTION

Mobile communications technology has enjoyed substantial growth over thepast decade. Many cars, trucks, airplanes, boats, and other vehicles areequipped with devices that allow convenient and reliable mobilecommunication through a network of satellite-based or land-basedtransceivers. Advances in this technology have also led to widespreaduse of hand-held, portable mobile communications devices.

Many customers of mobile communications systems also require an accuratedetermination of their position, and perhaps reporting of this positionto a remote location. For example, a cellular telephone in a vehicle orcarried by a person offers a convenient communication link to reportposition information. The position information may be generated bytraditional positioning systems, including a satellite-based positioningsystem such as the global positioning system (GPS), or a land-basedpositioning system, such as LORAN-C. These approaches, however, may notbe suitable for particular applications that require great positionaccuracy.

SUMMARY OF THE INVENTION

In accordance with the present invention, the disadvantages and problemsassociated with previous techniques used to locate and report theposition of a vehicle, person, or object equipped with a mobilecommunications device have been substantially reduced or eliminated. Oneaspect of the present invention provides a differential positioningsystem that integrates positioning technology with an existing mobilecommunications infrastructure.

According to an embodiment of the present invention, a locating systemusing a cellular telephone network and a positioning system includes areference positioning receiver having known position coordinates. Thereference positioning receiver receives first position signals from thepositioning system and generates correction data in response to thefirst position signals and the known position coordinates. A transmittersite of the cellular telephone network is coupled to the referencepositioning receiver and transmits the correction data generated by thereference positioning receiver. A mobile unit in communication with thecellular telephone network and the positioning system receivescorrection data transmitted by the transmitter site. The mobile unitalso receives second position signals from the positioning system anddetermines the location of the mobile unit in response to the secondposition signals and the correction data.

According to another embodiment of the present invention, a system forlocating a mobile unit within the service area of a mobilecommunications network includes a plurality of transmitter sites havingknown position coordinates, each transmitter site broadcastingtime-of-arrival (TOA) data. A mobile communications device on the mobileunit receives the TOA data transmitted by at least three transmittersites. A memory on the mobile unit stores known position coordinates ofthe transmitter sites. A processor receives the TOA data from the mobilecommunications device and determines the position of the mobile unit inresponse to the TOA data received from the transmitter sites and theknown position coordinates of the transmitter sites stored in thememory.

Important technical advantages of the present invention includeimproving the accuracy of existing positioning systems using a mobilecommunications system. In particular, existing transmitter sites of amobile communications network may be used as reference points totransmit position correction data to mobile units within the mobilecommunications network service area. Other important technicaladvantages include integration of communicating, locating, and reportingfunctions for an overall reduction in the cost and complexity of thesystem. For example, a differential GPS (DGPS) positioning system mayuse an existing communications link, such as the overhead message streamof a cellular telephone network, to send correction data from thetransmitter site to the mobile unit. Important technical advantages mayalso include accurate and immediate position fixes without relying oncalculations performed at a remote location. Other important technicaladvantages may also include implementation of a time-of-arrival (TOA)positioning system within the mobile communications network withoutland-based or satellite-based positioning technology. Other technicaladvantages are readily apparent to one skilled in the art from thefollowing figures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther features and advantages, reference is now made to the followingdescription taken in conjunction with the accompanying drawings, whereinlike reference numerals represent like parts, in which:

FIG. 1 illustrates a differential positioning system;

FIG. 2 illustrates an alternative embodiment of the differentialpositioning system of FIG. 1;

FIG. 3 is a schematic representation of a transmitter site associatedwith a reference positioning receiver;

FIG. 4 is a schematic representation of a mobile unit;

FIG. 5 is a schematic representation of a central host; and

FIG. 6 illustrates an alternative positioning system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates several components used in a differential positioningsystem 10. The system includes components of a satellite-based orland-based positioning system 12 and components of a mobilecommunications network 14. Differential positioning system 10 providesaccurate and immediate position information to vehicle 16 equipped witha mobile unit 17.

Positioning system 12 is illustrated as a satellite-based radionavigation system, such as the NAVSTAR positioning system (GPS). Thedescription uses the NAVSTAR GPS as a representative positioning system12, but any land-based or satellite-based system may be used. Forexample, positioning system 12 may be a land-based LORAN-C, aspace-based GLONASS, or any other appropriate positioning technology. Ingeneral, positioning system 12 comprises a plurality of space-based orland-based transmitters that emit position signals.

The NAVSTAR GPS consists of a number of satellites in approximatelytwelve hour, inclined orbits of the earth, each satellite transmittingposition signals. The GPS concept of operation is based upon satelliteranging. With position signals from three satellites, a GPS receiver canmake an accurate calculation of its position in three dimensions. Tomake a valid position fix, the GPS receiver measures the propagationtimes of position signals from the satellites to a very high accuracy.This is accomplished by synchronizing the transmission of positionsignals to an atomic clock. However, to reduce costs and complexity, theGPS receiver may not maintain such an accurate clock, which introduces aclock bias (C_(B)) between the satellite clock and the GPS receiverclock. By measuring the apparent satellite signal propagation times fromfour satellites rather than three, the redundancy can be used to solveC_(B). The signal propagation times correspond to ranges of the GPSreceiver from the satellites, related by the speed of light. Prior tocorrection for the clock bias C_(B), the apparent ranges of thesatellites are all in error by a fixed amount and are calledpseudoranges.

Two positioning services are provided by the NAVSTAR GPS. The precisepositioning service (PPS) which is reserved for military use providesaccuracy to within twenty-one meters (2 drms). The statistical term “2drms” refers to a value that falls within two standard deviations (usingthe root-mean-squared method) of the sampled performance data mean.Therefore, a stated accuracy of twenty-one meters (2 drms) means thatthe position error has an error of less than twenty-one metersapproximately ninety-five percent of the time.

The standard positioning service (SPS) which is available for generaluse provides accuracy to within thirty meters (2 drms). However, the SPSsignal accuracy is intentionally degraded to protect U.S. nationalsecurity interests. This process, called selective availability,degrades the accuracy of SPS position fixes to within one hundred meters(2 drms). The SPS may be degraded in a number of ways, for example, byproviding slightly inaccurate satellite orbital data to the receivers orby dithering the ranging information. Certain applications requirebetter accuracy than provided by degraded SPS, SPS, or even PPS.

Differential GPS technology (DGPS) may provide location accuracies towithin three meters (2 drms). Such accuracies allow, for example,accurate positioning of a delivery truck on a street map or preciselocating for an in-vehicle navigation system. The precision of the GPSsystem is improved by broadcasting differential correction data to a GPSreceiver. A typical DGPS positioning system, such as the one implementedby the U.S. Coast Guard, uses known position coordinates of a referencestation to compute corrections to GPS parameters, error sources, andresultant positions. This correction data is transmitted to GPSreceivers to refine received position signals or computed position.

Traditional DGPS positioning systems require the user to carry both aGPS receiver and an additional communications device to receive thecorrection data. For example, the Coast Guard implementation requires amaritime radio beacon receiver to obtain GPS correction data. This CoastGuard system is described in a document entitled “Implementation of theU.S. Coast Guard's Differential GPS Navigation Service,” U.S.C.G.Headquarters, Office of Navigation Safety and Waterway Services, RadioNavigation Division, Jun. 28, 1993. Another system, described in U.S.Pat. No. 5,311,194, entitled “GPS Precision Approach and Landing Systemfor Aircraft” and issued to Brown, describes a differential GPSimplementation for use in a precision approach and landing system foraircraft. In this system, the aircraft is required to carry a broadbandGPS receiver with added functionality to receive pseudolite signals thatcontain the correction data.

Differential positioning system 10 in FIG. 1 implements the DGPS conceptusing positioning system 12 integrated with mobile communicationsnetwork 14 to accurately determine the location of vehicle 16.Differential positioning system 10 utilizes components of mobilecommunications network 14 as reference stations that provide correctiondata to vehicle 16 over an existing communications link, such as thecontrol channel, overhead message stream, or paging channel of acellular telephone network. Mobile communications network 14 may be acellular telephone network, specialized mobile radio (SMR), enhancedspecialized mobile radio (ESMR), a personal communications service(PCS), a satellite-based or land-based paging system, a, citizen's band(CB), a dedicated radio system, such as those used by police andfirefighters, or any other appropriate mobile communications technology.

Differential positioning system 10 is described with reference tolocation of vehicle 16. The present invention contemplates location ofall types of vehicles, including cars, trucks, airplanes, boats, barges,rail cars, truck trailers, or any other movable object that is desirableto locate or track. Furthermore, differential positioning system 10 canalso be used to accurately locate a person carrying a portable orhand-held mobile unit 17. Potential applications of this technology mayinclude delivery service dispatch, less-than-full-load (LTL) truckingapplications, in-vehicle navigation systems, surveying applications,collision avoidance, emergency location using mobile 911 services, orany other application requiring accurate positioning information of avehicle, object, or person.

Differential positioning system 10 provides a more accurate position fixthan currently available navigation services, and may provide thesefixes near instantaneously or “on the fly.” In some applications, lowfrequency and low accuracy updates are sufficient, but otherapplications may need better accuracy and higher frequency updates innear real-time. For example, a delivery truck may require accurate, highfrequency position fixes for in-vehicle navigation to locate a specificdelivery address or to provide real-time directions to the driver.Differential positioning system 10 may provide these high frequencyupdates without relying on off-vehicle computations prevalent inprevious DGPS implementations. In addition, the same delivery truck maysend lower frequency position reports to a remote location. Theseposition reports may be sent at fixed time intervals, on-demand, or as aresult of a predetermined reporting event. Differential positioningsystem 10 may provide both low and high frequency position fixes andreports in such a hybrid navigation and position reporting system.

Satellite-based positioning system 12 is a navigation system usingNAVSTAR GPS, GLONASS, or other satellite-based or land-based radionavigation system to provide ranging data to mobile unit 17. Satellites18, 20, 22 maintain accurate and synchronized time and simultaneouslytransmit position signals that contain satellite specific and systeminformation required by mobile unit 17 to generate position fixes. Theposition signals transmitted by satellites 18, 20, 22 may include highprecision clock and ephemeris data for a particular satellite, lowprecision clock and ephemeris (called “almanac”) data for everysatellite in the constellation, health and configuration status for allsatellites, user text messages, and parameters describing the offsetbetween GPS system time and UTC.

Mobile unit 17 receives position signals over message data streams 26,28, 30 from satellites 18, 20, 22, respectively. Additional satellites(not shown) may also communicate message data streams to mobile unit 17.Typically, mobile unit 17 receives at least four satellite message datastreams to solve for position information independent of inherent clockbias (C_(B)) between positioning system 12 and mobile unit 17. Currentlythe NAVSTAR GPS system has twenty-one active satellites at 11,000 mileorbits of fifty-five degrees inclination with the equator. In normalconditions, mobile unit 17 may receive position signals from sevensatellites.

Using information from position signals 26, 28, 30 and optionallyadditional message data streams, mobile unit 17 may determine itsposition using accurate satellite position information transmitted bysatellites 18, 20, 22 and pseudorange data represented by the time ofarrival of message data streams 26, 28, 30 to mobile unit 17. Using SPSthis position fix may be accurate to within 30 meters (2 drms) or 100meters (2 drms) when selective availability degradation is activated. Ifmobile unit 17 is allowed to operate using PPS, then the position fixmay be accurate to within 21 meters (2 drms).

To provide a more accurate position fix for mobile unit 17, satellites18, 20, 22 also transmit message data streams 32, 34, 36, respectively,to a reference positioning receiver 38 on or in proximity to atransmitter site 40 of mobile communications network 14. Referencepositioning receiver 38 performs similar calculations to determine aposition fix from position signals received from satellites 18, 20, 22.Reference positioning receiver 38 compares the computed position fix toknown position coordinates and generates correction data fortransmission over correction data stream 44 to mobile unit 17 forfurther refinements of position fix provided by mobile positioningreceiver 24 (FIG. 4).

The known position coordinates of transmitter site 40 may be determinedby traditional surveying techniques. In addition, reference positioningreceiver 38 may perform position fixes over a statistically significantperiod of time to determine the known position coordinates. Filtering oraveraging position fixes by reference positioning receiver 38 over timeremoves or substantially reduces the effect of selective availabilitydegradation and may provide a more accurate position determination thanuncorrupted SPS or even PPS.

One type of correction data generated by reference positioning receiver38 is a position correction which is applied to the position fix ofmobile positioning receiver 24 (FIG. 4) of mobile unit 17 to achieve amore accurate position fix. The position correction may be inlatitude/longitude, compass direction and distance, or any otherappropriate coordinate system. When using a GPS positioning system 12,this technique provides accurate correction data when mobile unit 17 andreference positioning receiver 38 are located in a satellite common viewarea of approximately thirty square miles. In the satellite common viewarea all receivers operating in positioning system 12 receiveapproximately the same pseudorange errors assuming they are alllistening to the same group of satellites 18, 20, 22. This correctionmethod places less correction data in correction data stream 44 thanother methods, but the validity of those correction terms decreasesrapidly as the distance between mobile unit 17 and reference positioningreceiver 38 increases. The usefulness of this correction method isimpaired when mobile unit 17 and reference positioning receiver 38compute their position fixes using position signals from differentsatellites. Furthermore, this method requires that both mobile unit 17and reference positioning receiver 38 compute a navigation solution.

In an alternative correction method, reference positioning receiver 38computes pseudorange corrections (PRCs) to each satellite-18, 20, 22,which are then transmitted over correction data stream 44 to mobile unit17 to refine its navigation solution. The PRCs for satellites 18, 20, 22in view of reference positioning receiver 38 are the difference betweenthe pseudorange and the computed range to each satellite 18, 20, 22based on the known position coordinates of reference positioningreceiver 38. Each PRC message includes an identification of thesatellite 18, 20, 22 and a linear measure of the PRC. Although thismethod may include more transmission of data, it may result in a moreaccurate position fix. Furthermore, such a scheme provides additionalflexibility to allow mobile unit 17 to use navigation data from any ofthe satellites that reference positioning receiver 38 has furnishedPRCs.

An additional correction method generates position corrections based onpossible combinations of satellites 18, 20, 22 currently in view ofreference positioning receiver 38. This approach may be computationallyintensive at reference positioning receiver 38, but would allow for asimple adjustment of the solution computed by mobile unit 17. The numberof position corrections (PCs) may be computed using the followingformula:${{{No}.\quad {of}}\quad {PCs}}\quad = \frac{n!}{{r!}{\left( {n - r} \right)!}}$

where n is the number of satellites in the common view area and r is thenumber of satellites used in the position correction calculation. Forexample, for a position fix using four satellites and with sixsatellites in the satellite common view area, reference positioningreceiver 38 would have to generate fifteen PCs corresponding to fifteencombinations of four satellites each.

Each satellite 18, 20, 22 sends an identifier in its respective messagedata stream. Both mobile unit 17 and reference positioning receiver 38may use these identifiers to generate satellite group IDs (SGIDs) thatidentify the specific combination of satellites used for a position fix.Reference receiver 38 may generate the position correction for fifteencombinations (four satellites chosen from a total of six), and tag theposition corrections with the appropriate SGIDs. Mobile unit 17, havingdetermined an SGID for its position fix, may then choose the properposition correction identified by the same SGID to ensure that mobileunit 17 and reference positioning receiver 38 use the same combinationof satellites. Using this scheme with the NAVSTAR GPS, there would be10,626 unique SGIDs for satellite combinations of four out oftwenty-four satellites in the planned constellation.

The size and structure of a correction data message generated byreference positioning receiver 38 and transmitter over correction datastream 44 depends on the correction method employed and the precisionrequired. A single pseudorange correction (PRC) message for a satellitein the satellite common view area may include a satellite ID, the rangecorrection in a selected precision, and other associated portions of themessage, such as a header, delimiter, and checksum. A typical PRCmessage for six satellites described in the Motorola GPS TechnicalReference Manual (October 1993) is fifty-two bytes long, including theheader, delimiter, and checksum.

The size and structure of a single position correction message alsodepends on the precision required and the transmission protocol. Atypical position correction message may include a four byte SGID (1through 10,626), a one byte latitude correction, and a one bytelongitude correction. A multiple position correction message for fifteensatellite combinations (four satellites chosen from a total of six) maytotal 90 bytes of correction data. Appropriate header, delimiter andchecksum bytes consistent with the communication protocol of mobilecommunications network 14 may be added.

The precision of pseudorange or position corrections depends on theanticipated range of error and the number of bytes allocated to thecorrection data. For example, one byte of eight bits may providecorrection in the range of +/−127 meters with one meter bit resolution.One byte may also provide correction in 0.25 meter bit resolution over arange of approximately +/−32 meters. The precision, correction range,and byte allocation is a design choice that considers various factors,such as the available bandwidth in correction data stream 44, theaccuracy of the unrefined position fix at mobile unit 17, the correctionmethod employed, and the inherent inaccuracies of positioning system 12.

Correction data stream 44 allows correction data to be transmitted fromreference positioning receiver 38 to mobile unit 17. In one embodiment,correction data stream 44 may be the control channel, paging channel, oroverhead message stream currently implemented in cellular telephonetechnology. Currently, the control channel provides paging of incomingcalls, hand-off instructions, and other features of the cellulartelephone network, but may be modified by one skilled in the art toinclude transmission of correction data. Correction data stream 44 mayalso be implemented using any other communication link betweentransmitter site 40 and mobile communications device 42 (FIG. 4) inmobile unit 17, whether or not the communication link requires seizingof a voice or data channel.

There are several developing technologies that may provide a convenientimplementation of correction data stream 44. For example, cellulardigital packet data (CDPD) technology allows integration of data andvoice using the existing cellular telephone infrastructure. In a CDPDsystem, digital packets of data and analog voice segments share the samechannel. Other developments in digital cellular communications, such ascode division multiple access (CDMA) and time division multiple access(TDMA), allow digital data and digital voice signals to be interspersedon a communications channel. These technologies integrate digital datatransmission in a mobile communications network 14, and thereforeprovide a convenient implementation scheme for correction data stream44.

Using the technologies mentioned above or other appropriate digitalcommunications link, transmitter site 40 may either continuouslybroadcast correction data over correction data stream 44, such as in thecontrol channel of the cellular telephone network, or only sendcorrection data to mobile unit 17 when requested by a feature coderequest or by any other appropriate manner. Transmitter site 40 may sendcorrection data to mobile unit 17 in one large packet or in severalsmaller packets interspersed with other data used for mobilecommunications. The correction data may be packaged in existing, butunused, bytes of the control channel or in a dedicated protocol. Onepossible implementation would place correction data in the extendedprotocol described in the EIA/TIA-533 mobile communications standard,which provides for bidirectional communication between transmitter site40 and mobile unit 17.

Reference positioning receiver 38 may continuously receive positionupdates and continuously compute correction data for transmission tomobile unit 17 over correction data stream 44. Alternatively, referencepositioning receiver 38 may send correction data over correction datastream 44 at predetermined time intervals, at designated times whencorrection data stream 44 can accommodate the additional traffic, orwhen requested by mobile unit 17.

Reference positioning receiver 38 may include an additional capabilityto ensure that correction data transmitted to mobile unit 17 bytransmitter site 40 is current. This may be accomplished by including atime stamp in the correction data message to account for latency in thesystem. Using GPS technology as an example, satellites 18, 20, 22 inpositioning system 12 provide position navigation data each second.Reference positioning receiver 38 may include an additional byte thatindicates the delay in seconds of the correction data. The mobile unit17 may save time-stamped position signals and later synchronize andcorrect the position signals with the time-stamped correction datareceived from transmitter site 40. The post-processing to refine pastposition fixes may be performed by mobile positioning receiver 24 (FIG.4) or other separate processor in mobile unit 17.

Correction data stream 44 may be part of the control channel, part of aseized voice or data channel, or a separate channel requiring mobileunit 17 to re-tune to the correction data stream channel to receivevalid corrections for the area. Mobile unit 17 may continuously monitorcorrection data stream 44 transmitted from transmitter site 40.Furthermore, mobile unit 17 may alternately tune between severalcorrection data streams 44 from several transmitter sites 40 todetermine the strongest signal, usually relating to the nearesttransmitter site 40. This strongest channel select feature of mobileunit 17 assures that reference positioning receiver 38 and mobile unit17 will be in close proximity and receive position signals from the samegroup or nearly the same group of satellites 18, 20, 22. For a typicaltransmitter site spacing in a cellular telephone network, the distancebetween mobile unit 17 and reference positioning receiver 38 may be lessthan five miles, well within the satellite common view area of the GPSsystem.

Differential positioning system 10, as illustrated in FIG. 1,contemplates placing reference positioning receiver 38 on eachtransmitter site 40 within mobile communications network 14. When usingGPS technology as positioning system 12 and a cellular telephone networkas mobile communications network 14, the satellite common view area maybe much larger than the coverage area of a single transmitter site 40,thereby obviating the need to have reference positioning receivers 38 oneach transmitter site 40. For example, differential positioning system10 may include reference positioning receivers 38 on selectedtransmitter sites 40 of mobile communications network 14. In thisconfiguration, mobile unit 17, which may be capable of simultaneouslymonitoring correction data streams 44 from multiple transmitter sites40, may still receive correction data from a transmitter site 40 that iscurrently not providing communication service to mobile unit 17.Selected transmitter sites 40 equipped with reference positioningreceivers 38 may be spaced so that mobile unit 17 located anywhere inmobile communications network 14 can receive correction data ofsufficient signal strength from one of the selected transmitter sites 40equipped with reference positioning receivers 38.

FIG. 2 shows an alternative embodiment of differential positioningsystem 10 that places reference receivers 38 on selected transmittersites 40 in mobile communications network 14. As in FIG. 1, transmittersite 40 is associated with reference positioning receiver 38, whichreceives position signals in message data streams 32, 34, 36 fromsatellites 18, 20, 22, respectively. However, mobile unit 17 is locatedin an area serviced by transmitter site 46, which is not equipped withreference positioning receiver 38. Furthermore, mobile unit 17 is unableto receive correction data directly from transmitter site 40 due to theinability to monitor communications from transmitter sites 40 and 46,the distance from transmitter site 40, or other reasons. However, mobileunit 17 is close enough to reference positioning receiver 38 to receivenavigation data from at least a subset of satellites 18, 20, 22 servingreference positioning receiver 38. Using any of the correction methodsdescribed above with reference to FIG. 1, reference positioning receiver38 generates correction data and transmits this correction data throughlink 48 to transmitter site 46. Transmitter site 46 transmits correctiondata generated by reference positioning receiver 38 over correction datastream 44 to mobile unit 17. Mobile unit 17 uses the correction data torefine a position fix derived from position signals received fromsatellites 18, 20, 22 over message data streams 26, 28, 30.

Differential positioning system 10, illustrated in FIG. 2, reduces thenumber of reference positioning receivers 38 required by networkingcorrection data through link 48 between transmitter sites 40, 46. Link48 between transmitter sites 40, 46 may include microwavecommunications, bidirectional paging or control channels, directland-line connections, switching stations such as MTSOs, or any otherappropriate communications device to send correction data fromtransmitter site 40 to transmitter site 46.

FIG. 3 is a schematic representation of transmitter site 40 associatedwith reference positioning receiver 38. Reference positioning receiver38 may be mounted directly on transmitter site 40 or on a separatestructure or mounting. Reference positioning receiver 38 includes anantenna 50, receiver 51, controller 52, and memory 54. The followingdescription relates to the operation of reference positioning receiver38 with a GPS positioning system, however, the same concepts apply toother land-based and satellite-based positioning systems.

Reference positioning receiver 38 receives position signals in messagedata streams 32, 34, 36 from satellites 18, 20, 22, respectively. Theposition signals include navigation data, such as ephemeris, almanac,and clock correction data. Ephemeris data includes detailed informationabout the specific satellite course over the next two hours, the almanacdata includes less detailed information about the complete satelliteconstellation for a longer period, and the clock correction dataincludes information to correct for clock errors. The satellitetransmissions received by antenna 50 consist of a direct sequence spreadspectrum signal containing the ephemeris, almanac, and clock correctiondata at a rate of fifty bits per second. In the case of the SPS, apseudorandom noise signal with a chip rate of 1.023 MHz that is uniqueto each satellite is used to spread the spectrum of the informationwhich is then transmitted on a center frequency of 1575.42 MHz.

Receiver 51 receives satellite position signals having a bandwidth ofapproximately 2 MHz and a signal-to-noise ratio of approximately −20 dB.The relative movement between satellites 18, 20, 22 and referencepositioning receiver 38 causes an additional Doppler frequency offsetfrom the GPS center frequency. To recover the navigation data andmeasure the propagation time of the satellite position signals, receiver51 must cancel or allow for the Doppler frequency offset and generatethe proper coarse/acquisition code associated with each satellite 18,20, 22 to despread the signal. Once synchronization with thepseudorandom noise signal is achieved, receiver 51 may extract theephemeris, almanac, and clock correction data and pass this informationto controller 52.

Controller 52 receives navigation data from at least three satellitesand uses this information to determine a navigation solution based onwell-known triangulation techniques. In a four satellite fix, with eachsatellite position represented by coordinates (X_(n), Y_(n), Z_(n)) withthe indice n equal to one through four, the position coordinates (X, Y,Z) of reference positioning receiver 38 may be determined by solving thefollowing equations:

(X ₁ −X)²+(Y ₁ −Y)²+(Z ₁ −Z)²=(R ₁ −C _(B))²

(X ₂ −X)²+(Y ₂ −Y)²+(Z ₂ −Z)²=(R ₂ −C _(B))²

(X ₃ −X)²+(Y ₃ −Y)²+(Z ₃ −Z)²=(R ₃ −C _(B))²

(X ₄ −X)²+(Y ₄ −Y)²+(Z ₄ −Z)²=(R ₄ −C _(B))²

where R₁, R₂, R₃, R₄ are pseudorange measurements from the satellitesand C_(B) is a common clock bias. Controller 52 may use certain datastored in memory 54 to arrive at a navigation solution. Controller 52may then compare the instantaneous navigation solution (X, Y, Z) toknown position coordinates (X₀, Y₀, Z₀) stored in memory 54 to generateposition correction data in latitude/longitude, compass direction anddistance, or other appropriate coordinate system.

In an alternative embodiment, controller 52 may receive ephemeris,almanac, and clock correction data from satellites 18, 20, 22 andcompute a pseudorange (R_(N)) for each satellite. Since the satellitesignal contains information on the precise satellite orbits andcontroller 52 has known position coordinates (X₀, Y₀, Z₀) stored inmemory 54, the true range to each satellite 18, 20, 22 can becalculated. By comparing the true range and the measured pseudorange, apseudorange correction (PRC) for each satellite 18, 20, 22 may becomputed and sent as correction data. As described above with referenceto FIG. 1, controller 52 may also provide position correction data basedon navigation solutions using all possible combinations of satellites18, 20, 22 currently in view of reference positioning receiver 38.

Correction data in any of the various forms described above is sent bycontroller 52 to channel controller 56 of transmitter site 40 overcommunication link 58. Communication link 58 may be a direct wireconnection, a radio communication link, a connection through a switchedtelephone system, or other appropriate communication link. Depending onthe configuration of differential positioning system 10, channelcontroller 56 may send correction data to radio duplexer 60 fortransmission over transmitter site antenna 62 to mobile unit 17.Alternatively, channel controller 56 may pass correction data throughlink 48 to transmitter site 46 currently serving mobile unit 17.

Also shown in FIG. 3 as part of transmitter site 40 are time-of-arrival(TOA) data generator 64 and clock 66 that may be used in an alternativepositioning system 200 described with reference to FIG. 6. TOA datagenerator 64 generates a TOA data message and sends this message tochannel controller 56 for transmission to mobile unit 17 overtransmitter site antenna 62. The TOA data message may include a precisetime of transmission based on information maintained by clock 66. Clock66 and TOA data generator 64 are shown as elements of transmitter site40, but it should be understood that their functions may also beimplemented in a central or distributed device accessible by transmittersites 40, 46 of mobile communications network 14.

FIG. 4 is a schematic representation of a mobile unit 17 that includesmobile positioning receiver 24, mobile communications device 42, andother associated hardware and software, described below. Mobilepositioning receiver 24 is similar in construction and function toreference positioning receiver 38 and includes an antenna 82, receiver84, controller 86, and memory 88. In operation, mobile positioningreceiver 24 receives position signals from satellites 18, 20, 22 overmessage data streams 26, 28, 30 at antenna 82. Receiver 84 processesthese signals to extract ephemeris, almanac, and clock correction data.Controller 86 receives this information and computes a navigationsolution or pseudorange measurements. These calculations performed bycontroller 86 may use data stored in memory 88.

Mobile communications device 42 includes an antenna 90, transceiver 92,and hand set 94. In operation, mobile communications device 42 receivescorrection data at antenna 90 over correction data stream 44. Thecorrection data may be transmitted directly from transmitter site 40equipped with reference positioning receiver 38 as described withreference to FIG. 1, or indirectly through link 48 and transmitter site46 as described with reference to FIG. 2. As described above, thecorrection data may be in a variety of forms, including single ormultiple position corrections, or pseudorange corrections to eachsatellite. Correction data is then stripped from correction data stream44 by transceiver 92. Correction data may be passed to processor 100over link 95 or over any other appropriate path, such as through busdrivers 112 and modem or dual tone multifrequency (DTMF) coder/decoder110. Hand set 94 provides traditional voice or data communication usingmobile communications device 42.

Processor 100 manages the communicating, locating, and reportingfeatures of mobile unit 17. Processor 100 receives a navigation solutionor pseudorange measurements from controller 86 and correction data fromtransceiver 92. Coupled to processor 100 is memory 102 which may containprograms, databases, and other information required by processor 100 toperform its functions. For example, memory 102 may contain a table ofknown position coordinates of transmitter sites 40 for use in computingthe position of mobile unit 17 in the alternative positioning system 200described with reference to FIG. 6. Memory 102 may be random accessmemory (RAM), read-only memory (ROM), CD-ROM, removable memory devices,or any other device that allows storage or retrieval of data.

Processor 100 and controller 86, as well as memory 102 and memory 88,may be separate or integral components of mobile unit 17. For example,controller 86 may include a port that directly receives correction dataand allows mobile positioning receiver 24 to output a refined positionfix. Mobile unit 17 contemplates any arrangement, processing capability,or task assignment between controller 86 and processor 100.

In operation, processor 100 generates a refined position fix for mobileunit 17 based on the navigation solution or pseudorange measurementsfrom controller 86 and the correction data from transceiver 92. Thisrefined position fix may be sent to output device 104 to generate amoving or static display of vehicle 16 on a map represented by map datastored in memory 102. Alternatively, output device 104 may produceaudible information, such as directions or location updates, to theoperator of vehicle 16.

Processor 100 is also coupled to input device 106 that allows operationof mobile unit 17. Input device 106 may be a keypad or touch screen, aswell as voice recognition software and hardware that can accept audiblecommands and information. Furthermore, both output device 104 and inputdevice 106 may include fixed or removable storage media, such asmagnetic computer discs, CD-ROM, or other suitable media to both receiveoutput and provide input to processor 100.

Processor 100 may also generate data messages for transmission to aremote location using mobile communications device 42. The data messagesmay include the refined position fix of mobile unit 17, the time ofreporting, or information input by the vehicle operator, as well as anyother information collected by processor 100 from various sensors 108.For example, sensors 108 may include various engine sensors, trucktrailer sensors, security monitors, or other devices generatinginformation on the status or condition of mobile unit 17, vehicle 16, orits operator. The generation and transmission of a data message may bebased on elapsed time, movement of mobile unit 17, sensor readings, orany other piece of information that may necessitate reporting to aremote location. The data messages are sent from processor 100 throughmodem or DTMF coder/decoder 110 to bus drivers 112, and then totransceiver 92 for transmission over antenna 90 to a remote location,such as central host 120 (FIG. 5). Data messages may also be sentdirectly to transceiver 92 over link 95.

Mobile unit 17 may also include a clock 116 coupled to processor 100that may be used to synchronize the navigation solutions or pseudorangemeasurements received from controller 86 with latent correction datareceived from transceiver 92. Clock 116 may also be used in alternativepositioning system 200 described with reference to FIG. 6. In operation,clock 116 provides accurate time to processor 100, and may receive clockcorrection updates from mobile positioning receiver 24 or throughcorrection data from mobile communications device 42.

Components of mobile unit 17 shown in FIG. 4 may be packaged into one ormore housings. Mobile unit 17 may be mounted to vehicle 16 or an objectto be tracked. Mobile unit 17 may also be packaged as a portable,hand-held device that provides personal locating, communicating, andreporting functions. For example, a portable, hand-held mobile unit 17may be used by surveyors, rescue teams, individuals that may changeforms of transportation, or any other application requiring portabilityof mobile unit 17.

FIG. 5 is a schematic representation of a central host 120. Central host120 receives communications from mobile unit 17, such as reportsgenerated by processor 100, through link 122. Link 122 may be one or acombination of dedicated telephone lines, switched telephone lines,microwave communications links, satellite-based communications links, orany other suitable communication link that allows mobile unit 17 totransmit data to or receive data from central host 120.

A data message from mobile unit 17 enters central host 120 through amodem or DTMF coder/decoder 124 and passes to central controller 126.Coupled to central controller 126 is memory 128 and input/output device130. Memory 128 may be RAM, ROM, CD-ROM, removable memory devices, orany other device that allows storage or retrieval of data. Input/output130 includes any variety of output devices, such as a display, a speakerto provide audible information, removable storage media, or any otherappropriate output device. Input/output device 130 may also include avariety of input devices, such as a keyboard, mouse, touch screen,removable storage media, or any other appropriate input device.

Central controller 126 receives data messages from mobile unit 17 andprocesses this information to locate, track, dispatch, and communicatewith mobile unit 17. For example, central controller 126 can maintain adatabase in memory 128 of all mobile units 17 with their currentlocation, status, and relevant sensor readings. This database can alsobe used to initiate communication with mobile unit 17. Furthermore,central controller 126 may perform a call delivery function that routesincoming calls to mobile unit 17 through link 122. This aspect of calldelivery is fully described in application Ser. No. 08/095,166, entitled“Method and Apparatus for a Nation-Wide Cellular Telephone Network”filed Jul. 20, 1993, and application Ser. No. 08/175,256 entitled “DataMessaging in a Communications Network” filed Dec. 28, 1993, bothapplications commonly owned by the assignee of the present application,and both applications hereby incorporated by reference.

FIG. 6 illustrates an alternative positioning system 200 that utilizesequipment of the existing mobile communications network 14 to locatevehicle 16 equipped with a modified mobile unit 17. Mobile unit 17communicates with transmitter sites 202, 204, 206 over communicationslinks 208, 210, 212, respectively. Communication links 208, 210, 212 maybe the control channel, overhead message stream, or paging channel of acellular telephone network, a portion or all of a seized voice or datachannel, or a dedicated channel. Transmitter sites 202, 204, 206 may becoupled to a network in a variety of ways. For example, transmitter site202 is coupled to transmitter site 204 over land-line connectionsthrough MTSO 214. Transmitter site 202 is coupled to transmitter site206 over a microwave or other radio link 216. Transmitter site 204 iscoupled to transmitter site 206 over a direct or dedicated connection218.

Positioning system 200 operates in a similar fashion to an aspect ofdifferential positioning system 10 described with reference to FIGS. 1and 2, but does not rely on a positioning system 12 to transmitnavigation data. Instead, transmitter sites 202, 204, 206 transmittime-of-arrival (TOA) data over respective communications links 208,210, 212. Mobile unit 17 receives TOA data and computes the position ofmobile unit 17 using the TOA data and known position coordinates oftransmitter sites 202, 204, 206.

The TOA data from transmitter sites 202, 204, 206 may be transmitted ina variety of ways. In one method, a network clock 220 synchronizes theinstantaneous transmission of TOA data from transmitter sites 202, 204,206. Using this method, the time of reception at mobile unit 17 providespseudorange measurements to transmitter sites 202, 204, 206. As indifferential positioning system 10 of FIGS. 1 and 2, a fourthtransmitter site allows the position of mobile unit 17 to be computedwithout regard for a clock bias (C_(B)) between network clock 220 andclock 116 (FIG. 4) maintained on mobile unit 17.

In another embodiment, transmitter sites 202, 204, 206 transmit TOA dataat different times, but include the time of transmission in the messageto mobile unit 17. Assuming cellular transmitter sites 202, 204, 206maintain synchronized time through network clock 220, mobile unit 17 cangenerate pseudorange measurements by comparing the message time ofarrival to the time of transmission.

Transmitter sites 202, 204, 206 and mobile unit 17 may have differentconfigurations when operating in positioning system 200. Referring toFIG. 3, transmitter site 40 does not need an associated referencepositioning receiver 38 to provide location information in positioningsystem 200. Transmitter site 40, however, does include TOA datagenerator 64 and clock 66 to generate the TOA data for transmission tomobile unit 17. Referring now to FIG. 4, mobile unit 17 does not requiremobile positioning receiver 24 for operation within positioning system200. TOA data is received by transceiver 92 and sent to processor 100,which uses the TOA data to compute pseudoranges to cellular transmittersites 202, 204, 206. Using well-known triangulation techniques describedwith reference to FIG. 3, processor 100 may then compute a position fixof mobile unit 17 using the pseudoranges and known position coordinatesof transmitter sites 202, 204, 206 stored in memory 102.

Although the present invention has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art, and it is intended that the present inventionencompass such changes and modifications as fall within the scope of theappended claims.

What is claimed is:
 1. A system for locating a mobile unit within theservice area of a mobile communications network, comprising: a pluralityof transmitter sites operable to transmit time-of-arrival dataindependent of periodic timed pulses, wherein the time-of-arrival datacontains the precise time of transmission of the time-of-arrival datafrom the transmitter sites; a mobile communications device on the mobileunit and operable to receive time-of-arrival data transmitted by atleast three transmitter sites; a memory operable to store known positioncoordinates of the transmitter sites; and a processor coupled to themobile communications device and the memory, the processor operable todetermine the position of the mobile unit in response to thetime-of-arrival data and the known position coordinates of thetransmitter sites.
 2. The system of claim 1, wherein each transmittersite transmits time-of-arrival data using a control channel of themobile communications network.
 3. The system of claim 1, wherein thetransmitter sites are associated with a cellular telephone system. 4.The system of claim 1, wherein the transmitter sites simultaneouslytransmit time-of-arrival data.
 5. The system of claim 1, wherein theprocessor is operable to determine the position of the mobile unit usingtriangulation techniques.
 6. The system of claim 1, wherein thetransmitter sites furnish time-of-arrival data in response to a requestby the mobile unit.
 7. The system of claim 1, wherein the processor isfurther operable to: determine the time of reception of thetime-of-arrival data by the mobile communications device; determinepseudorange measurements based upon the difference between the time oftransmission and the time of reception of the time-of-arrival data; anddetermine the position of the mobile unit based upon the pseudorangemeasurements.
 8. The system of claim 1, further comprising a clockcoupled to the transmitter sites, the clock operable to synchronize thetransmission of time-of-arrival data from the transmitter sites.
 9. Asystem for locating a mobile unit within the service area of a mobilecommunications network, comprising: a plurality of transmitter sites,each transmitter site operable to transmit time-of-arrival dataindependent of periodic timed pulses and mown position coordinatesassociated with the transmitter site, wherein the time-of-arrival datacontains the precise time of transmission of the time-of-arrival datafrom the transmitter sites; a mobile communications device on the mobileunit and operable to receive time-of-arrival data and known positioncoordinates transmitted by at least three transmitter sites; and aprocessor coupled to the mobile communications device, the processoroperable to receive the known position coordinates from the mobilecommunications device, the processor further operable to determine theposition of the mobile unit in response to the time-of-arrival data andthe known position coordinates.
 10. The system of claim 9, wherein eachtransmitter site transmits time-of-arrival data and known positioncoordinates of the transmitter site using a control channel of themobile communications network.
 11. The system of claim 9, wherein thetransmitter sites are associated with a cellular telephone system. 12.The system of claim 9, wherein the transmitter sites simultaneouslytransmit time-of-arrival data and known position coordinates.
 13. Thesystem of claim 9, wherein the controller is operable to determine theposition of the mobile unit using triangulation techniques.
 14. Thesystem of claim 9, wherein the transmitter sites furnish time-of-arrivaldata and known position coordinates in response to a request by themobile unit.
 15. The system of claim 9, wherein the processor is furtheroperable to: determine the time of reception of the time-of-arrival databy the mobile communications device; determine pseudorange measurementsbased upon the difference between the time of transmission and the timeof reception of the time-of-arrival data; and determine the position ofthe mobile unit based upon the pseudorange measurements.
 16. The systemof claim 9, further comprising a clock coupled to the transmitter sites,the clock operable to synchronize the transmission of time-of-arrivaldata and known position coordinates from the transmitter sites.